1. Field of the Invention
This invention relates to navigation systems and more particularly to navigation systems which utilize frequency diversity transmission for the illumination of navigation markers which include retro-directive elements containing non-linear devices which re-radiate signals with frequencies that are linear combinations of the frequencies of the received signals, thus providing a means for identifying the markers.
2. Description of the Prior Art
Navigational systems have been proposed heretofore that employ retro-directive identifiable devices to mark channels or routes to be taken navigating vehicles. Some of the earlier systems such as that taught in U.S. Pat. No. 2,461,005, issued on Feb. 8, 1949 to G. C. Southworth and in U.S. Pat. No. 2,520,008, issued on Aug. 22, 1950 to A. P. King, provide an identification means by modulating the signal received at the marker prior to retransmitting the signal. However, these systems retransmit frequencies that have been originally transmitted and as such must compete with returns from background clutter at the receiver. In many environments the energy level of the background clutter at the receiver is such that the modulated reradiated signals may not be detected.
Later systems such as that taught in U.S. Pat. No. 3,518,546, issued on June 30, 1970 to A. A. Augenblick et al provide a non-linear element in the retro-directive device to generate harmonics of the frequency of the received signals and a means for modulating same. These harmonic signals are then reradiated towards a receiver located adjacent to the original transmitter. Since signals are reradiated at a harmonic frequency, rathen than at the fundamental or received frequency, separation from the fundamental signal returns of the background clutter is theoretically more easily accomplished at the receiver. However, a heterodyning receiver is usually a generator of harmonic signals and some portion of the background clutter fundamental signal is converted to harmonic signals tending to mask the desired externally generated harmonic signals. To minimize this possibility filters with extremely deep and steep passband characteristics are employed prior to mixing the received harmonic signals with the local oscillator signals. Further, stringent transmitter and local oscillator frequency control is required to maintain the desired signals within the filter and i.f. amplifier passbands.
The present invention is directed to an improved navigation system which utilizes retro-directive devices that reradiate signals which are identifiable with respect to normal signal returns from land, ice, rain and sea clutter. These signals are readily detected and facilitate the identification of route markers in a high clutter environment.